The present invention relates to a connected body and an optical transceiver module, and more particularly to a connected body and an optical transceiver module which are mounted by a face down bonding.
A recent high-speed transmission line starts the movement of applying optical fibers instead of electric wirings for the reasons that (1) the transmission line is wideband, (2) the transmission line is excellent in the electromagnetic wave noise resistance, and (3) the transmission line is small in the volume of fibers and light in weight. In the optical wiring, one of the most important factors is an optical coupling structure of an optical element such as a semiconductor laser or a photodiode with an optical transmission line such as an optical fiber or an optical waveguide. In order to obtain a high coupling efficiency, the optical element and the optical transmission line are required to provide the mounting precision of several tens μm in positioning even in the case of the multimode transmission. Also, after a reliability test of a temperature cycle or a high-temperature and high-humidity has been conducted on the optical element and the optical transmission line, no displacement or separation must occur. On the other hand, it is a basic premise that the optical wiring is inexpensive from the viewpoint of a substitute for the electric wiring. Therefore, the initial costs of material and a fabrication process must be suppressed as much as possible.
As a method of mounting the optical element which meets the above requirements, there is a method in which a surface receiving/emitting elements of a surface emitting laser (VCSEL: vertical cavity surface-emitting laser) and a surface illuminated type photodiode are mounted on a substrate by flip chip bonding, so as to be optically coupled with the optical transmission line that is located on a lower portion of the substrate, as disclosed in JP-A No. 2005-164801. With the above configuration, it is possible to form a joint structure of the optical element in the same process as that in the case of mounting the flip chip in the conventional electronic circuit.
Also, an annular electrode is formed on the light receiving/emitting portion to conduct flip chip bonding, as disclosed in JP-A No. 2003-298167. As a result, a loss due to the cross talk of the optical signal and the inflow of the underfill resin is prevented from increasing.
Further, JP-A No. 2006-091241 discloses a photoelectrical complex wiring component that enables information transmission which is excellent in noise tolerance, high in speed, and high in quality.
As a technique in which the optical element disclosed in JP-A No. 2006-091241 is mounted by face down, there are Au bump connection and solder connection. However, JP-A No. 2006-091241 fails to teach that there arise the following problems.
(1) In order to optically couple the optical element with the optical transmission line with high efficiency, it is essential that the parallelism between the optical element and the substrate can be precisely controlled. However, it is difficult to provide parallelization. More particularly, in the case where an Au bump is used for junction of the electrodes, a variation in the height of the bump leading end is extremely large. As a result, it is necessary to introduce a process for uniforming the heights of the respective bumps.
(2) The electrodes of the optical element is formed at intervals of several hundreds μm with about 100 μmφ, which is smaller than the electrodes of a general electronic circuit component. Accordingly, the supply quantity of joining material is small, and the control of the supply quality is difficult. For that reason, in the case where the joining material is excessively supplied, there arise problems such as an increase in the loss that is caused by the outflux of the joining material into the optical path at the time of mounting the optical element, or short-circuiting between electrodes.
(3) A distance between the optical element and the optical transmission line is an important factor for determining the optical coupling efficiency of the system. This distance is determined according to the joining material that joins the optical element and the optical transmission line. That is, the existence of the joining material increases the distance between the optical element and the optical transmission line. Also, the joining conditions and the variation of the supply quantity cause the optical output of the emitting element, and the deterioration and fluctuation of the sensitivity of the receiving element.